| With regard to the harmful effects of residual element tin(Sn)on mechanical properties in low alloy structural steels,this dissertation mainly studies the harmfulness of residual element tin on the properties of low alloy structural steel 33MnCrB5-2 and explores the feasibility of improving it through the intercritical heat treatment process.Firstly,the effects of different tin contents(steel 1#-without addition,steel 2#-0.03wt%,steel 3#-0.06wt%,steel 4#-0.09wt%)on the continuous cooling phase transformations of under-cooled austenite,the hot ductility and mechanical properties of 33MnCrB5-2 steel are systematically studied.In order to solve the problem of mismatch between strength and toughness of steel with residual element tin,the intercritical heat treatment process is introduced to study the effect of this process on the phase transformation,microstructure and properties of the experimental steels with different tin contents and the mechanism of reducing the harmfulness of tin.The continuous cooling phase transformations and the critical points of tincontaining 33MnCrB5-2 steel were studied by thermal dilatometer.The experimental results showed the phase transformations of 33MnCrB5-2 steel in the cooling rate range of 0.2～50℃·s-1 included ferrite(F)transformation,pearlite(P)transformation,bainite(B)transformation and martensite(M)transformation.When the cooling rates was in the range of 0.2～0.5 ℃·s-1,the microstructure was composed of F and P.With the increase of cooling rate,the ferrite content decreased gradually.When the cooling rate was 1 ℃·s-1,B transformation occurred,and the mixed products of B,P and F were obtained.When the cooling rate was 2℃·s-1,M started to appear.When the cooling rate exceeded 10℃·s-1,B disappeared and all the transformation product was M.Compared with base experimental steel 1#,the F start transition temperatures of the experimental steels with tin contents of 0.06wt%and 0.09wt%were increased,while the M start transition temperatures were decreased by 8℃ and 21℃,respectively.When the tin content was not more than 0.06wt%,it had little effect on the critical point Ac3.When the tin content was 0.09wt%,the temperature of Ac3 point was 11℃ higher than that of the base experimental steel 1#.In addition,0.09wt%tin had no obvious effect on the austenite grain size of the experimental steel.Gleeble-3800 thermal simulation testing machine was used to carry out high temperature tensile test on the experimental steel with tin.The experimental results showed that when the tensile temperature was between 700℃ and 900℃,with the increase of tin content,the peak stress of the experimental steel increased,while the hot ductility decreased significantly,which was due to the non-equilibrium segregation of tin which reduced the grain boundary energy,weakened the adhesion between grains,promoted the sliding of grain boundaries,accelerated the formation and growth of grain boundary micropores,and worsened the hot ductility.When the tensile temperature was at 950℃,the hot ductility of the experimental steels increased significantly due to the occurrence of dynamic recrystallization.At this time,the deterioration resulting from tin on the hot ductility of the experimental steel was weakened.The experimental steels were subjected to conventional heat treatment(quenching and tempering).The experimental results showed that good strength and toughness matching was achieved for the base experimental steel 1#without tin addition,whose tensile strength was 1275 MPa,elongation after fracture was 13.8%,and Charpy impact energy(V-notch)was 76 J.Due to the solid solution strengthening effect of tin atoms,0.03,0.06 and 0.09wt%tin contents increased the tensile strength of the experimental steels by 6,9 and 39 MPa,respectively.Due to the grain boundary segregation of tin atoms,0.03,0.06 and 0.09wt%tin contents reduced the impact energy values of the experimental steels by 24,30 and 46%,respectively.In addition,for the experimental steel with 0.09wt%tin content,the crack propagated along the grain boundaries and went deep into the sample.The microstructure of F-M was obtained by intercritical heat treatment of the experimental steel.The impact energy values of the normalizing experimental steels subjected to intercritical heat treatment(IQ-T)were all about 30 J,and the fracture surface belonged to brittle fracture characterized by cleavages.The impact energy values of the quenching experimental steels subjected to intercritical heat treatment(Q-IQ-T)were more than 80 J,and the fracture belonged to the ductile fracture characterized by dimples.It was found that the ferrite in the IQ-T treated experimental steel was irregular block,while the ferrite in the Q-IQ-T treated experimental steel was strip.Massive ferrite accelerated the crack propagation,while the microstructure of strip ferrite was beneficial to retard crack propagation.In addition,the proportion of high angle grain boundaries in the Q-IQ-T samples was higher than that in the IQ-T samples,which was beneficial to the Q-IQ-T process to improve the toughness of experimental steel.Therefore,the quenching state was the optimal pretreatment before intercritical quenching.The experimental steel pretreated as quenched were treated at different intercritical quenching temperatures.The results showed that the F content decreased gradually with the increase of intercritical quenching temperature.Taking steel 4#as an example,when the intercritical quenching temperatures were 760,770 and 780℃,the ferrite contents were 55.3,32.7 and 3.6%,respectively.The ferrite content determined the strength and toughness of the experimental steel to a certain extent.Since the strength of the experimental steel obtained by intercritical heat treatment at 760℃ was low,and the toughness obtained by intercritical heat treatment at 780℃ was low,the optimal intercritical quenching temperature was 770℃.After being intercritical heat treated at 770℃,0.03,0.06 and 0.09wt%Sn contents reduced the impact energy values of the experimental steels by 19,17 and 21%respectively,which indicated that the deterioration effect of residual element tin on the impact toughness of the experimental steel was greatly alleviated.In addition,compared with the experimental steel with conventional heat treatment,the impact toughness values of the experimental steels 1-4#with different tin contents were increased by 26,34,51 and 85%,respectively.This was mainly due to the fact that the microstructure obtained by the intercritical treatment contained a certain amount of ferrite,which provided a certain degree of toughness.Simultaneously,the existence of ferrite also changed the distribution of tin elements。The grain size after the intercritical treatment was smaller,and the grain boundaries increased,which helped to reduce the degree of tin segregation at the grain boundaries,thus alleviating the deterioration of tin on the toughness of the experimental steels,and finally making the experimental steels with tin obtain a good strength and toughness match.The experimental steel was heated to the intercritical temperature interval with the martensite obtained in the quenched state as the initial structure.The prior austenite grain boundaries and all boundaries of the martensite structure provided many effective sites for austenite(γ)nucleation.At 730℃ isothermal for 60 min,globular γ was observed to form at the prior austenite grain boundaries.With the increase of isothermal time to 180 min or the increase of isothermal temperature to 740℃,acicular γ was also observed to be distributed in the martensite block boundaries and lath boundaries.However,in the isothermal process at 760℃,only globular γ was observed and acicular y was not observed,which indicated that the high temperature region was more conducive to the formation of globular y.This could effectively avoid the occurrence of micro structure genetics and was conducive to the refinement of microstructure.In addition,the presence of 0.09wt%tin had no obvious effect on the morphology and nucleation position of austenite. |
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